1. Field of Invention
This invention related generally to an improvement of audio coding method, and more particularly, to a method for analyzing the consistency of signal energy to determine the time to perform block switching for better audio compression efficiency.
2. Description of Related Art
Perceptual audio coding is widely used in every music box related product currently. One of the crucial technologies in perceptual audio coding is the reduction of pre-echo phenomena. The solution in prior art is to divide a frame of signals into several blocks, and then choose long-type window coding or short-type window coding according to characteristics among these blocks. The characteristic is, the corresponding frame is more suitable for long-type window coding for increasing coding compression ratio when audio signal is stationary. FIG. 1 is a block diagram illustrating the conventional perceptual audio coding. The block diagram of FIG. 1 is well known to the art, thus related descriptions are omitted herein.
Conventionally, there are many different schemes to determine the suitable type for each frame. These documents related to block switching are listed as follows:
1. U.S. Pat. No. 5,299,239 (SONY 1994), which calculates energy of signals in different time intervals and performs block switching if the difference between two energies exceeds a specific constant. This method is too simple to find a correct timing to do block switching. The easy-changing characteristics of audio signal requires more complex algorithm to guarantee an appropriate tracking.
2. ISO/IEC 13818-7, “Information Technology-Generic Coding of Moving Pictures and Associated Audio, Part 7: Advanced Audio Coding”, which determines the time of block switching by calculating perceptual entropy in psychoacoustic analysis. This method requires too much computation power and highly depends on the accuracy of psychoacoustic analysis.
3. ATSC A/52 “ATSC Digital Audio Compression Standard (AC-3)”, which divides a large-scale signal output by a high-passing filter into several small-scale data frames. It then locates the maximum of sample values in each data frame and compares those maximum values. The block switching is activated if the difference of maximum values among the neighboring data frames exceeds some specific constants. Nevertheless, this method is too poor to resist the interference of noise, thus its accuracy is not stable.
4. M. J. Smithers et al., “Increased Efficiency MPEG-2 AAC Encoding”, which is similar to the above method 3, but the high-passing filter coefficients and segmentation resolution are adjusted adaptively according to the sampling frequency.
5. U.S. Pat. No. 5,451,954 (DOLBY 1995), which is also similar to the above method 3, but the high-passing filter is replaced by a band-passing one. Besides, the average of the largest three samples in the data frame is selected to replace the maximum sample in the method 3 for further comparison between neighboring data frames.
As noted, the above mentioned methods 3, 4 and 5 can not handle noise interference well. The poor interference resistance by merely adopting single filter and constant threshold to trigger the block switching mechanism makes those methods insufficiently to deal with the instationary characteristics of audio signal.
Furthermore, the above method 2 uses an exhausted method to seek the best choice for block switching, i.e. executing every possible block switching decision to evaluate their effectiveness. The whole mechanism is computation-intensive and raising the hardware cost to an unbearable degree without any acceptable quality guarantee.
As mentioned above, the prior arts decide when to do block switching mainly by identifying the existence of transient data. That is, mainly depends on locating the energy maximum in blocks to decide to do block switching or not. Nevertheless, because of the in-stationary characteristic of audio signal, it's deficient to use the local energy maximum in blocks as a judgment to do block switching. Besides, an abrupt change of energy cannot explain all the happening of smearing effect. Any prior art whose principle is analyzing the transient nature of signal may suffer the inefficient ability to do block switching.
We can conclude from all these disadvantages mentioned above that the prior arts are no competitive in application. They have many difficulties in applying to practical commerce. Cost of practice is too high and can not reach an agreeable compromise with performance.